Acids and Bases

Dissociation of Strong Bases  Strong bases are metallic hydroxides  Group I hydroxides (NaOH, KOH) are very soluble  Group II hydroxides (Ca, Ba, Mg, Sr) are less soluble  pH of strong bases is calculated directly from the concentration of the base in solution MOH(s)  M + (aq) + OH - (aq)

Reaction of Weak Bases with Water The base reacts with water, producing its conjugate acid and hydroxide ion: CH 3 NH 2 + H 2 O  CH 3 NH OH - K b = 4.38 x 10 -4

K b for Some Common Weak Bases BaseFormula Conjugate Acid KbKb Ammonia NH 3 NH x Methylamine CH 3 NH 2 CH 3 NH x Ethylamine C 2 H 5 NH 2 C 2 H 5 NH x Diethylamine (C 2 H 5 ) 2 NH (C 2 H 5 ) 2 NH x Triethylamine (C 2 H 5 ) 3 N (C 2 H 5 ) 3 NH x Hydroxylamine HONH 2 HONH x HydrazineH 2 NNH 2 H 2 NNH x Aniline C 6 H 5 NH 2 C 6 H 5 NH x Pyridine C 5 H 5 N C 5 H 5 NH x Many students struggle with identifying weak bases and their conjugate acids.What patterns do you see that may help you?

Reaction of Weak Bases with Water The generic reaction for a base reacting with water, producing its conjugate acid and hydroxide ion: B + H 2 O  BH + + OH - (Yes, all weak bases do this – DO NOT endeavor to make this complicated!)

A Weak Base Equilibrium Problem What is the pH of a 0.50 M solution of ammonia, NH 3, K b = 1.8 x ? Step #1: Write the equation for the reaction NH 3 + H 2 O  NH OH -

A Weak Base Equilibrium Problem What is the pH of a 0.50 M solution of ammonia, NH 3, K b = 1.8 x ? Step #2: ICE it! I C E x +x x xx NH 3 + H 2 O  NH OH -

A Weak Base Equilibrium Problem Step #3: Set up the law of mass action xxx E What is the pH of a 0.50 M solution of ammonia, NH 3, K b = 1.8 x ? NH 3 + H 2 O  NH OH -

A Weak Base Equilibrium Problem Step #4: Solve for x, which is also [OH - ] xxx E [OH - ] = 3.0 x M NH 3 + H 2 O  NH OH - What is the pH of a 0.50 M solution of ammonia, NH 3, K b = 1.8 x ?

A Weak Base Equilibrium Problem Step #5: Convert [OH - ] to pH xxx E What is the pH of a 0.50 M solution of ammonia, NH 3, K b = 1.8 x ? NH 3 + H 2 O  NH OH -

Read 14.6: Bases Page 706 questions #71,72,79,80,83,84